CHAP. XII.] THE VASCULAR SYSTEM. 145 



amount from time to time, but under normal circumstances, 

 never exceeds certain limits. Under abnormal conditions, these 

 limits may be exceeded, and the result is known as oadema or 

 dropsy. Similar excessive accumulations may also occur in the 

 larger lymph-spaces, the serous cavities. 



The possible causes of oedema are, on the one hand, an ob- 

 struction to the flow of lymph from the lymph-spaces, and on 

 the other hand, an excessive trans udation, the lymph gathering 

 in the lymph-spaces faster than it can be carried away by a 

 normal flow. (Edema is almost always due to the latter cause, 

 viz. excessive transudation. 



The inflammatory oedema, due to changes in the walls of the 

 blood-vessels, we have already touched on in speaking of the 

 capillary circulation. In this kind of oedema the transudation 

 is, besides being crowded with migrating corpuscles, more dis- 

 tinctly coagulable than ordinary lymph. Allied to this inflam- 

 matory oedema is the " effusion," which appears in the serous 

 cavities when they are inflamed, as in pleurisy and peritonitis. 



Functions of the lymph. The lymph derived from the blood 

 delivers to the elements of the tissues the material each element 

 needs to maintain its functional activity, and returns to the 

 blood the products of this activity, which products may be 

 simple waste, or matters capable of being made use of by some 

 other tissue. There is thus a continual interchange going on 

 between the blood and the lymph. How this interchange 

 is effected may be partially understood by the following 

 illustration. 



If a tumbler be completely divided vertically into two com- 

 partments by a moist piece of membrane, and a watery solution 

 of common salt be placed in one compartment, and a watery 

 solution of sugar in the other, it will be found after a time that 

 some of the salt has found its way into the solution of sugar, 

 and, vice versa, some of the sugar into the salt solution. Such 

 an interchange is said to be due to diffusion ; and if the process 

 were allowed to go on for some hours, the same proportion of 

 salt and sugar would be found in the solutions on each side 

 of the dividing membrane. So in the living body. The lymph, 

 originally like the blood-plasma (it is blood-plasma forced to 

 transude through the capillaries by the pressure of the blood), 

 becomes altered by the metabolic changes of the tissues which 



